1. Field of the Invention
The invention relates to the chemical vapor deposition processes used in the fabrication of semiconductor devices, and more particularly to methods for sequential chemical vapor deposition of thin films.
2. Description of the Related Art
Chemical vapor deposition (CVD) is one of the principal methods used in semiconductor fabrication to form a variety of thin films, including dielectric and conductive layers. In chemical vapor deposition, a material layer is formed by the reaction of gas phase reactants at or near a substrate surface. Various methods of energy input can be used to promote the chemical reaction, including thermal energy, photons, and plasma. In a plasma-assisted CVD process, the reaction is promoted by activating the gas phase reactants to form radical species, electrons and ions, which form a plasma. Background gases which participate in forming a plasma, but may or may not participate in the deposition reaction may also be included in plasma-assisted CVD processes. The properties of the deposited material, as well as the rate at which material is deposited may depend on precursor gases to the gas phase reactants, on the background gases, and on the plasma conditions.
Advances in semiconductor devices increase the demands for chemical vapor deposition of highly conformal and uniform thin films. In conventional CVD processes, it is often difficult to obtain a thin film that is conformal across the entire surface of a wafer, particularly when the wafer has a large radius. Furthermore, in conventional CVD processes, reaction byproducts often get trapped in the film being grown, affecting the purity and the electrical properties of the film.
U.S. Pat. No. 4,058,430 of Suntola et al. describes an atomic layer epitaxy (ALE) process for growing compound thin films. In the ALE process of the '430 patent, a substrate is exposed to the vapor of a first element in a reaction chamber and the substrate is heated to a temperature sufficiently high to induce a chemical reaction between the first element and the substrate for forming a single atomic layer of the first element on the substrate surface. The heated substrate can be subsequently exposed to the vapor of a second element to form a single atomic layer of the second element on top of the atomic layer of the first element. The process is repeated until the compound film reaches a desired thickness. U.S. Pat. No. 4,398,973 also of Suntola et al. describes an alternate method of performing the ALE process to the '430 patent which uses gaseous compounds to supply the reactive elements. These techniques for ALE processes relate more closely to CVD processes.
In both the '430 and '973 patents, the ALE process is driven by thermal energy through heating the substrate. Specifically, the temperature of the substrate has to be high enough to prevent condensation of the elements on the surface of the substrate and to induce chemical reaction between the reactive elements and the substrate surface. A disadvantage of the ALE process of the '430 and '973 patents is that when forming compound thin films, the different reactive elements must have overlapping temperature windows for forming a saturated layer in order to perform the ALE process. Thus, the ALE process of the '430 and '973 patents has limited applications.
U.S. Pat. No. 5,916,365 describes a sequential chemical vapor deposition process where a first reactant forms a saturated layer on the surface of a substrate and a second reactant is passed through a radical generator which activates the second reactant into a gaseous radical. The gaseous radical of the second reactant is used to induce a chemical reaction of the saturated layer. An disadvantage of the process disclosed in the '365 patent is that the reactive species in the CVD process are limited to free radical species that are generated by the radical generator. Radical generators that can be used in the '365 process generally use plasma discharges to break up most of the bonds in the precursor gas. Thus, the radical generation process is non-selective and all types of radicals, whether beneficial or harmful to the deposition process of interest, are generated. Furthermore, radical generators also suffer from contamination problems as some gases tend to cause a film to be deposited on the walls of the generators.
It is desirable to provide a sequential chemical vapor deposition process with improved process characteristics and also capable of forming a variety of compound thin films with improved film properties.